F. A. Mullen. Left-click photo to enlarge.
A little detective work reveals that the diver is Frank Aloysius Mullen, a diving team member of the New York Athletic Club. This NY Times article mentions Mullen. It also says that the summer home of the club was Travers Island (Pelham Manor).
I found a post card that depicts what appears to be the same diving platform at low tide. Look below the fold.